1. Field of the Invention
The invention relates to optical amplifiers and lasers. More particularly, the invention relates to optical amplifier and laser apparatus and methods using cascaded Raman resonators.
2. Description of the Related Art
Optical amplifiers and lasers are used within optical communications systems to compensate for losses incurred throughout the system. Optical amplifiers often include a Raman amplifier or laser to pump light at a particular wavelength. See, e.g., U.S. Pat. No. 5,323,404, which is assigned to the instant assignee and hereby is incorporated by reference herein.
In general, Raman amplifiers and Raman lasers are based on stimulated Raman scattering, a non-linear optical process that involves converting light from an optical source to the vibrational modes of a non-linear optical transmission medium (e.g, an optical fiber, typically a silica-based optical fiber) and re-radiation at a different (typically longer) wavelength.
For example, a cascaded Raman laser typically is a Raman laser with a non-linear optical transmission medium that has, in addition to a pair of reflectors that defines an optical cavity for radiation of an output wavelength .lambda..sub.n, at least one Raman-Stokes order reflector pair defining a corresponding optical cavity for radiation of wavelength .lambda..sub.n-1 &lt;.lambda..sub.n, where n.gtoreq.2. The reflector pairs are, e.g., Bragg gratings, etched gratings or in-line refractive index gratings. When fused silica is used as the non-linear medium, the maximum Raman gain occurs at a frequency shift of 13.2 terahertz (THz), which corresponds to a wavelength shift of approximately 50-100 nanometers (nm) for pump wavelengths between approximately 1.0 and 1.5 microns (.mu.m).
A cascaded Raman resonator (CRR) includes a non-linear optical transmission medium to generate Raman laser energy at a specific output wavelength (.lambda..sub.n). More specifically, the cascaded Raman resonator converts light from an optical source such as a pump laser operating at a pump wavelength (.lambda..sub.p) to the desired output wavelength (.lambda..sub.n). Suitable applications of such cascaded Raman resonator include, e.g., remotely pumped erbium (Er) fiber amplifiers in repeaterless optical fiber communication systems.
However, conventional cascaded Raman resonators typically require optical sources that operate at a specific pump wavelength (.lambda..sub.p) depending on the cascaded Raman resonator output wavelength (.lambda..sub.n) desired. For example, a cascaded Raman resonator having an output wavelength (.lambda..sub.n) of 1480 nm typically is useful only with an optical source such as a pump laser operating at a pump wavelength (.lambda..sub.p) of 1117 nm, which corresponds to a series of resonators spaced at wavelengths corresponding to the maximum Raman gains or frequency shifts of about 13.5 THz. Similarly, a cascaded Raman resonator having an output wavelength (.lambda..sub.n) of 1450 nm typically is useful only with an optical source such as a pump laser operating at a pump wavelength (.lambda..sub.p) of 1100 nm.
Thus, it would be desirable to have available Raman laser devices that are power scaleable and more independent of the device input wavelength (.lambda..sub.p). Such devices would be more versatile in that, e.g., the devices would not be limited to use with sources having only a specific pump wavelength (.lambda..sub.p) that corresponds to a Raman-Stokes order that leads to the desired output wavelength (.lambda..sub.n) of the device.